Implemented pre/post commands and --build=ddox + support "sourceFiles". ...

Other fixes:
 - fixed class Dub to work with other path than cwd
 - "files" has been deprecated in favor of "sourceFiles"
 - --build=docs also works now
 - Changed the way build flags are parsed (instead of checking every possible field for existence, all fields are iterated and their name is matched)

When the user calls install from the command line, a dependency will be added to the project's package.json. However, this will reformat that file. Should now perform more like user expectations mentioned in #12.

Fixed naming convention for enum InstallLocation and avoid reinstalling packages that are already up to date.

Fixed "dub upgrade" when a package is stalled user or system wide.

Renamed all the "vibe" package to "vibecompat" to avoid conflicts.

Another round of replacing invalid uses of to!string(Path) with Path.toNativeString().

Added rudimentary support for the --arch= option and moved build type flag generation to a common place.

Avoid use of deprecated functions + fixed package installation.

"dub build" and "dub run" are now also executed using the generator system (RdmdGenerator).

Implemented basic multi-compiler support (with only a dmd implementation right now).

Added @property to several getter functions with field names.

Handling missing package.json more gracefully (without a crash) + some trace log messages.

Fixed up to date check when checking for validity of an installed package.

Renamed Application to Project and put it into its own module + added a stub MonoD generator.